1. Field
The present invention relates to wireless communications. More particularly, the present invention relates to pseudo-random number (PN) generators for spread spectrum communications systems.
2. Background
The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communication in which a large number of system users are present. Although other multiple access communication system techniques are known in the art, such as time division multiple access (TDMA and GSM), frequency division multiple access (FDMA), and AM modulation schemes such as amplitude companded single sideband (ACSSB), the spread spectrum modulation technique of CDMA has significant advantages over these other modulation techniques for multiple access communications systems. The use of CDMA techniques in a multiple access communications system is disclosed in U.S. Pat. No. 4,901,307, issued Feb. 13, 1990, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS” and U.S. Pat. No. 5,103,459, issued Apr. 7, 1992, entitled “SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,” both assigned to the assignee of the present invention and incorporated herein by reference.
CDMA systems are typically designed to conform to a particular CDMA standard. Examples of such CDMA standards include the “TIA/EIA/IS-95-A Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System” (hereinafter, the IS-95-A standard) and the TIA/EIA/IS-98-A, -B, and -C standards entitled “Recommended Minimum Performance Standard for Dual-Mode Spread Spectrum Cellular and PCS Mobile Stations.”
CDMA systems are direct sequence spread spectrum systems that spectrally spread the transmitted data over an entire available system bandwidth with a set of inphase and quadrature pseudo-random noise (PN) sequences. The PN sequences are selected because of certain important “randomness” properties that provide superior performance. Each CDMA standard defines the specific PN sequences to be used for spreading the data.
In CDMA systems, a particular geographic area can be partitioned into a number of neighboring cells and each cell can be further partitioned into a number of sectors. Each cell or sector is serviced by a base station that communicates with a number of mobile stations located within or near the cell or sector coverage area. The forward link refers to transmissions from the base station to the mobile station and the reverse link refers to transmissions from the mobile station to the base station. The forward link and the reverse link are allocated separate frequencies.
For a CDMA system that conforms to the IS-95-A standard, each transmitting base station is assigned a particular offset of the PN sequences. Specifically, in accordance with the IS-95-A standard, the base stations are each assigned one of 512 possible offsets. The assigned offset allows the mobile station to identify each base station with which it communicates.
On the forward link, a pilot signal is typically transmitted by the base station and used by the mobile station for acquisition. For an IS-95-A compliant system, the pilot signal is simply a transmission of the PN sequences at the assigned offset. The pilot signal enables the mobile station to acquire a local base station in a timely manner. The mobile station also derives synchronization information and relative signal power information from the received pilot signal.
As demand for wireless communication increases, a geographic area may include multiple spread spectrum systems. For example, the geographic area may be simultaneously serviced by one CDMA system operated at a cellular frequency band and another CDMA system operated at a Personal Communications System (PCS) (or another) frequency band. The mobile station may be designed with the capability to acquire and communicate with one or more CDMA systems. During the acquisition process, if the base stations of these different CDMA systems transmit using similar PN sequences, the mobile station may not be able to easily distinguish the pilot signals from these systems. As a result, additional signal processing may be required to acquire and identify the desired system, which can prolong the acquisition process.
Thus, techniques that aid in the detection and acquisition of a particular CDMA system in a multiple systems environment are highly desirable.